Activity coefficient COSMO

Pumam, R., Taylor, R., Klamt, A., Eckert, F., Schiller, M. Prediction of infinite dilution activity coefficients using COSMO-RS. Ind. Eng. Chem. Res. 2003, 42, 3535-3541. 

Solubility modelling with activity coefficient methods is an under-utilized tool in the pharmaceutical sector. Within the last few years there have been several new developments that have increased the capabilities of these techniques. The NRTL-SAC model is a flexible new addition to the predictive armory and new software that facilitates local fitting of UNIFAC groups for Pharmaceutical molecules offers an interesting alternative. Quantum chemistry approaches like COSMO-RS [25] and COSMO-SAC [26] may allow realistic ab-initio calculations to be performed, although computational requirements are still restrictive in many corporate environments. Solubility modelling has an important role to play in the efficient development and fundamental understanding of pharmaceutical crystallization processes. The application of these methods to industrially relevant problems, and the development of new... 

It is very satisfying and useful that the COSMO-RS model—in contrast to empirical group contribution models—is able to access the gas phase in addition to the liquid state. This allows for the prediction of vapor pressures and solvation free energies. Also, the large amount of accurate, temperature-dependent vapor pressure data can be used for the parameterization of COSMO-RS. On the other hand, the fundamental difference between the liquid state and gas phase limits the accuracy of vapor pressure prediction, while accurate, pure compound vapor pressure data are available for most chemical compounds. Therefore, it is preferable to use experimental vapor pressures in combination with calculated activity coefficients for vapor-liquid equilibria predictions in most practical applications. 

Although COSMO-RS generally provides good predictions of chemical potentials and activity coefficients of molecules in liquids, its accuracy in many cases is not sufficient for the simulation of chemical processes and plants, because even small deviations can have large effects on the behavior of a complex process. Therefore, the chemical engineer typically prefers to use empirical thermodynamic models, such as the UNIQUAC and NRTL, for the description of liquid-phase activity coefficients with... 

As a typical example from industrial practice we consider the simulation of a process with the reaction of methylphosphinic acid and butanol to methylphosphinic acid butyl ester and water, which was modeled by Gordana Hofmann-Jovic at InfraServ Knapsack [C28]. Because of the lack of experimental data for the binary systems with phosphorous compounds, COSMO-RS was used for the prediction of the binary activity coefficients. Then the results were fitted by an NRTL equation and the entire process was modeled by a commercial process simulator. The resulting phase diagrams were in close agreement with experimental measurements obtained later (Fig. 8.2). 

Technically, COSMO-RS meets all requirements for a thermodynamic model in a process simulation. It is able to evaluate the activity coefficients of the components at a given mixture composition vector, x, and temperature, T. As shown in Appendix C of [Cl 7], even the analytic derivatives of the activity coefficients with respect to temperature and composition, which Eire required in many process simulation programs for most efficient process optimization, can be evaluated within the COSMO-RS framework. Within the COSMOt/ierra program these analytic derivatives Eire available at negligible additionEd expense. COSMOt/ierra can Eilso be csdled as a subroutine, Euid hence a simulator program can request the activity coefficients and derivatives whenever it needs such input. 

One of the concerns regarding the use of COSMO-RS thermodynamics directly in simulations is the considerably larger computation time that is required for the evaluation of the activity coefficients compared to simpler empiricEd models with... 

The application of COSMO-RS to the calculation of infinite-dilution activity coefficients in ionic liquids was surprisingly successful. As shown in Fig. 8.5, the activity coefficients of neutral compounds in ionic liquids are very well described. This was achieved without any special adjustment of COSMO-RS, which was developed and parameterized for neutral solvents, just by describing the ionic liquid as a 50 50 mixture of anions and cations. We only needed to take into account the convention of chemical engineers of counting a pair of an anion and a cation as... 

Indeed, prediction of the change of the equilibrium constants of a chemical reaction in a variable liquid environment requires nothing other than the prediction of the chemical potentials or activity coefficients of the starting materials and products in the liquids. Thus, this task can be performed simply by using the standard COSMO-RS capabilities. Successful applications of this... 

C26 M. Diedenhofen, F. Eckert and A. Klamt, Prediction of infinite dilution activity coefficients of organic compounds in ionic liquids using COSMO-RS, J. Chem. Eng. Data, 48 (2003) 475-479. 

At present, molecular simulation consumes too much CPU time to be used directly as a means for general-purpose property estimation, including phase equilibrium. However, recent advances with the COSMO-RS and COSMO-SAC models allow prediction of activity coefficients from electron density profiles that are computed from quantum chemistry methods for molecules. If libraries of electron density profiles for molecules can be made public, these models can provide an alternative predictive technique to UNIFAC. However, the quality of the predictions still needs to be improved. 

A recent alternative to group-contribution activity-coefficient estimation methods is based on interactions between surface charge distributions (determined by quantum-mechanical calculations) of molecules in solution. The solvation model used for the charge-distribution calculation is known as COSMO the most widely used method based on this technique is called COSMO-RS [47]. 

Task-specific ionic liquids are of special interest in process intensification. The great variety of existing ionic liquids leads to an unavoidable theoretical screening at the beginning of a project. To date, the only fast and predictive virtual screening method is COSMO-RS, which provides activity coefficients for the relevant species in a set of ionic liquids. From these, selectivites and capacities are then calculated. Figure 2.4 depicts data from the open literature [13] to illustrate the performance of COSMO-RS. 

As the IL is described by a binary mixture of anions and cations, the resulting j,cosMO-RS jg ternary value. The activity coefficient can be calculated at any molar concentration including infinite dilution. In case of infinite dilution and for univalent ions (applies for almost aU IL), the ternary value can be corrected to a binary value (which is also the result of the experiments) by Eq. (9.10). 

COSMO-RS calculations are performed for different ILs other than [HMIM][NTf2]. Figure 9.10 shows the experimental and the predicted activity coefficients of ethanol in various ILs at 333 K. The prediction is accurate for all [NTf2]-ILs. The chain length variation at the imidazolium cation is described correctly. Also the methylation of the C2 is predicted properly. 

Furthermore, COSMO-RS is known to perform well for predictions of activity coefficients and solubilities in solvent mixtures [14,23] and in most cases reproduces solubility maxima correctly. A challenging problem case, however, the solubility of paracetamol in a water-dioxane mixture, is shown in Figure 9.3. Experimental data for the system paracetamol/water-dioxane have been taken from three different sources [31-33] and compared with COSMOt/term computations at the TZVP and the newer FINE parameterization [6]. 

C4mpyr]PF6) based on the combination of various scalar parameters. This trend was in accordance with that of infinite dilution activity coefficients of pyridine plus thiophene in ILs p redicted by quantum chemical based conductor like screening model-real solvents (COSMO-RS) method. The CH-x interaction between the CH group in the imidazolium cation and the aromatic ring in the sulfur-compound was considered as the main interaction between imidazolium ILs and sulfur-compounds (Ananthar Banerjee, 2011). 

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